Mount a Filesystem. But you'll have to investigate which drive or partition has your root partition. Mount a disk to /mnt, check if its the root partition. Once you find the root partition, you'll want to read its /etc/fstab - which will be /mnt/etc/fstab when mounted - to check for other partitions you may need to mount. You absolutely need to mount any separate boot if they exist.

You should probably also start reading the Arch Installation Guide because you're literally repeating steps from this guide to repair your system. Specifically, you are on step 1.11, then you'll want to do 3.2, 3.6, and 3.8. 3.8 blows up into the Boot Loader guide, which leads to the guide for GRUB. I think you only need to do GRUB 1.1 to fix your system. But your should read everything in between. Some instructions won't make sense if you dont.